const singers = [
    { name: '许嵩', num: 200 },
    { name: '周杰伦', num: 266 },
    { name: '林俊杰', num: 201 },
    { name: '陈奕迅' },
    { name: '薛之谦' },
]


singers.forEach((item, index, arr) => {
    // console.log(arr);
})

Array.prototype.my_forEach = function(callback) {
    for(let i = 0; i < this.length; i++) {
        callback(this[i], i, this)
    }
}

singers.my_forEach((item, index, arr) => {
    console.log(item, index);
})